As cemented carbides which have widely been used for cutting metals, there has hitherto been known a WC—Co alloy comprising a hard phase containing WC as a principal component and a bond phase made of a metal of the iron group such as cobalt, or an alloy obtained by adding carbides, nitrides or carbonitrides of metals of the groups 4a, 5a and 6a in the Periodic Table to the WC—Co alloy. In the latter case, grains of the solid solution of WC and carbides, nitrides and carbonitrides of metals of the groups 4a, 5a and 6a in the Periodic Table are added to the hard phase and bond phase.
These cemented carbides are principally utilized as a cutting tool for cutting cast irons and carbon steels, and are also utilized extensively for cutting stainless steels, recently. Stainless steels have been used in various fields because of characteristics such as excellent corrosion resistance, oxidation resistance and heat resistance, and the amount of them to be machined have been increased every year.
However, it has been known that stainless steels are typical hardly machinable materials because of properties such as easy occurrence of work hardening, low thermal conductivity and high affinity with tool materials.
Among WC cemented carbides for cutting tool, cemented carbides classified into so-called M series in accordance with JIS B 4053 (1996) are generally used to cut stainless steels. WC—TiC—Ta(Nb)C—Co cemented carbides are principally used in M series, and further TiC and Ta(Nb)C are added in a comparatively small amount to provide the cutting tool with the toughness.
However, even in case the stainless steel is cut by using a cutting tool made of a conventional cemented carbide of M series, it is hard to satisfactorily cut for a long time because of large wear amount of the cutting tool and short service life of the tool.
In addition, severe plastic deformation at the primary boundary portion is caused by a cutting resistance from the machined surface that is work-hardened during the cutting of the stainless steel, thus reducing service life of the tool.